266 THE BELL SYSTEM TECHNICAL JOURNAL, APRIL 1951 



The use of deposited metal on lacquered paper for the capacitor elements 

 provides very small, but dependable capacitors at low cost.'^ • ^ For example, 

 the 2 mf element is only J" wide x 1|" long by ye" thick, which is approxi- 

 mately one quarter the volume of the older type. The small size of these 

 capacitor elements is the chief factor in making possible a small, compact 

 network with common impregnation of the parts. Their self-healing char- 

 acteristic should largely eliminate service failure from breakdown. 



A feature of the network is the use of an autotransformer in the sidetone 

 balancing circuit. This element provides inductance and, through the use of 

 a short-circuited winding, resistance in the balancing circuit. It also serves 

 to couple a resistance and capacitor to this circuit at the correct impedance 

 level. The improved sidetone balance over the range of subscriber loop 

 conditions, which was mentioned earUer, is accomplished by carefully pro- 

 p)ortioning these impedance elements. 



In the interests of universahty, a split primary winding is used in the 

 induction coil. Contacts are provided in the switch for closing both sides of 

 the line and for disconnecting the ringing circuit when required. These 

 features permit a simple conversion of the circuit in the field for the various 

 individual and party line services for which it is intended. 



A filter is provided in the network which serves to increase the dial 

 contact life as well as to suppress radio frequency induction. 



Switch 



In previous designs, the weight of the handset was sufficient to operate 

 the switch through a direct linkage acting against the force of the contact 

 springs. Because of the lighter handset of the new set, it is impractical to 

 utihze such an arrangement. In the new switch, the activating force is fur- 

 nished by a coil spring, and the contact springs are biased to oppose this 

 force, thus acting as a counterbalance. The handset weight, through lever 

 arms, is thus required to overcome only the force differential between the 

 coil spring and the contact springs. 



The contact springs are positioned relatively by means of a stationary 

 notched detail or card which maintains the proper spring separations and 

 sequences. The operating springs are actuated by a second card which is 

 coupled to the lever arm in such a manner as to become disengaged when the 

 contact springs reach the end of their travel. This permits the lever arm and 

 plungers to travel in excess of the amount required for contact operation 

 with reasonable length of springs thus accommodating a wide variation in 

 dimensional tolerances, and eliminating the need for hand adjustment of the 

 springs during assembly in manufacture. 



At the point where the contact springs reach the end of their stroke and 

 the activating card disengages, the full force of the coil spring comes into 



